Cable television networks, including community antenna television (CATV), hybrid fiber-coaxial (HFC), and fiber networks, have been in widespread use for many years and are extensive. The extensive and complex cable networks are often difficult for a cable operator to manage and monitor. A typical cable network generally contains a headend, which is usually connected to several nodes that provide bi-directional content to a cable modem termination system (CMTS). In many instances, several nodes may serve a particular area of a town or city and several customer premises are connected to each of the nodes. The CMTS contains several receivers, and each receiver connects to modems of many of the customers. For instance, a single receiver may be connected to hundreds of modems at customer premises. Data may be transmitted downstream to the modems on different frequency bands. The modems communicate to the CMTS via upstream communications on a dedicated frequency band, referred to as a return band.
Cable networks are also increasingly carrying signals, which require a high quality and reliability of service, such as Voice over IP (VoIP) communications, streaming video, etc. Any disruption of voice or data traffic is a great inconvenience and often unacceptable to a customer for these type of services. Various factors may affect the quality of service, including the quality of the upstream channels. One factor that affects the quality of upstream communications is the presence of linear distortion impairments, such as micro-reflections (MRs) of communication signals, group delay variation (GDV), and amplitude distortion (AD).
These linear distortion impairments are known to be mitigated by the fundamental digital communications receiver function of equalization. During equalization, an equalizer generates coefficient information that is used to create a digital filter, with an inverse channel response, canceling distortion in the channel caused by the linear distortion impairments. The equalization coefficients in Data Over Cable Service Interface Specification (DOCSIS) 2.0 and DOCSIS 3.0 are 24 symbol-spaced coefficients (also referred to as equalizer taps). Equalization is part of virtually all modern telecommunications platforms, and is instrumental in proper upstream communications for all DOCSIS systems.
However, achieving equalization within the context of other network impairments such as Gaussian noise, ingress and impulse noise, signal clipping, etc. is not trivial. These non-linear impairments impede the ability of the equalizer to measure linear distortion and derive the equalizing filter. As a result, the equalizer may become unstable which is typically manifested by a general rise of all equalization coefficients. When this happens, data communications are seriously impaired or may be lost completely.
Since each cable modem is located at a unique point within the network, the equalizing filter required for each modem is unique. For instance, a given MR may affect some modems while having no effect on other modems. In environments where moderate to significant noise is present the equalizing filters manifest problems of instability. Possible sources of noise include narrow ingress such as common path distortion (CPD), and wideband impulse noise. When significant noise exists on the upstream channel that the equalizer is not able to resolve, the equalizer derives equalizing filter updates for each cable modem, which not only reflect the linear distortion impairments, for example the MR, but also random, possibly non-linear, volatile noise. As these equalizing filter updates are made and sent to the cable modems, generally all of the secondary equalizer taps of the equalizer slowly climb in power until ultimately, the equalizing filter distorts modem transmission so much that communications are no longer supported with the CMTS.
Further, as is often the case when the equalizer is used, data is sent using higher modulations (16 quadrature amplitude modulation (QAM), 32 QAM, 64 QAM) while station maintenance (which is required by DOCSIS to maintain connectivity with the modem) uses a lower modulation quadrature phase shift keying (QPSK). The robustness of QPSK sometimes allows the modems to remain connected or registered while no longer being able to pass upstream data. Similarly, sometimes the modems will completely deregister and have to reregister before data communications may continue. Both cases result in significant impacts to an end user as data communications are impeded for a significant period.